(1) Technical Field
This invention relates to a method which varies the slurry solid content dispensed on a polishing pad for controlling the polishing rate for specific areas on a semiconductor wafer during planarizing and permits more accurate control of the polishing rate across the semiconductor surface while performing planarizing to produce a uniform substrate surface.
(2) Description of the Prior Art
The following documents relate to a method for controlling a polishing rate across a substrate surface when performing planarization.
U.S. Pat. No. 6,398,627B1 issued Jun. 4, 2002 to Chiou et al. describes a slurry dispenser having multiple adjustable nozzles.
U.S. Pat. No. 6,234,877B1 issued May 22, 2001 to Koos et al. shows a CMP tool with adjacent slurry and diluting solution dispensers.
U.S. Pat. No. 6,106,728 issued Aug. 22, 2000 to Iida et al. shows a CMP apparatus.
U.S. Pat. No. 5,658,185 issued Aug. 19, 1997 to Morgan III et al. shows another CMP apparatus.
The manufacture of an integrated circuit device requires the formation of various layers (both conductive and non-conductive) above a substrate to form the necessary components and interconnects. During the manufacturing process, certain layers or portions of layers must be removed in order to pattern and form the various components and interconnects. Chemical mechanical polishing (CMP) is the method of choice for planarization of a surface of a semiconductor wafer, such as a silicon wafer, at various stages of the integrated circuit processing. CMP is also used to flatten optical surfaces; metrology samples and in various metal and semiconductor based substrates.
CMP is a technique in which chemical slurry is used in conjunction with a mechanical polishing pad to polish away materials on a semiconductor wafer. The mechanical movement of the pad relative to the wafer (and in conjunction with the slurry) provides the abrasive force to polish the exposed surface of the wafer. In the most common form of CMP, a substrate is mounted on a polishing head, which rotates against a polishing pad placed on a rotating table. The mechanical force derives from the rotating table speed and the downward pressure on the head. The chemical slurry is constantly transferred under the polishing head. Rotation of the polishing head helps in the slurry delivery as well as in averaging the polishing rates across the substrate surface. A constant problem of CMP is that the polishing rate varies from the periphery to the center of the wafer for various reasons. Pad bounce is one reason. Variations in the velocity encountered in the rotational movement is another. Some amount of averaging is achieved by rotating the wafer but variations still result in non-uniform polishing across the wafer surface. It is an important goal in the CMP processing to try to minimize this inequality in polishing rates.
This invention is concerned with improving the difference in thickness between center and edge on a wafer. Many of the oxides that are deposited by plasma enhanced methods, and used for inter-metal dielectric are consistently thicker at the substrate edge. The topographical variation from center to edge presents a problem that necessitates improvement to these additive processes. The variation can be as high as one thousand angstroms. This difference imparts a challenge for oxide CMP to polish faster at the edge and slower at the center, so that post-CMP thickness uniformity is acceptable. A uniform film thickness across the wafer after oxide CMP is needed to achieve good printing of small features across the wafer, and it will prevent yield loss issues such as missing vias of metal shorts.
The fabrication of integrated circuits on a semiconductor substrate involves a number of steps where patterns are transferred from photolithographic photo masks onto the substrate. Integrated circuits are typically formed on the substrates by the sequential deposition of conductive, semi conductive or insulative layers. Discriminating etching of the layers assisted by photolithography creates specific structures and devices. Precise focusing for high-resolution photolithographic exposure yields well defined and highly integrated circuit structures.
During the forming of these well-defined integrated circuit structures, it has become increasingly important to construct line widths measuring in the sub micron and nanomicron ranges. The photolithographic processing steps opens selected areas to be exposed on the substrate for subsequent processes such as oxidation, etching, metal deposition, and the like, providing continuing miniaturization of circuit structures. Each of the metal layers is typically separated from another metal layer by an insulation layer, such as an oxide layer. Therefore, there is a need to polish the substrate's constructed surface to provide a planar reference. Planarization effectively polishes away non-planar entities. To enhance the quality of an overlying layer, one without discontinuities of other blemishes, it is imperative to provide an underlying surface for the structured layer that is free of scratches and is ideally planar.
Conventionally, during the fabrication of integrated circuit structures, planarizing of the overlying structured layer is accomplished by CMP. The uniform removal of material from the patterned and non-patterned substrates is critical to substrate process yield. Generally, the substrate to be polished is mounted on a tooling head which holds the substrate using a combination of vacuum suction or other holding methods to contact the rear side of the substrate and a retaining lip or ring around the edge of the substrate to keep the substrate centered on the tooling head. The front side of the substrate, the side to be polished, is then contacted with an abrasive material such as a polishing pad or abrasive strip. The polishing pad or strip may have free abrasive fluid sprayed on it, abrasive particles affixed to it, or may have abrasive particles sprinkled on it.
The ideal substrate polishing method used by most semiconductor foundries is CMP. This choice is based on numerous factors which include; control of relative velocity between a rotating substrate and a rotating polishing pad, the applied pressure between substrate and polishing pad, choosing the polishing pad roughness and elasticity, and a uniform dispersion of abrasive particles in a chemical solution (slurry). In summary, the CMP process should provide a constant cutting velocity over the entire substrate surface, sufficient pad elasticity, and more importantly a controlled supply of clump-free polishing slurry.
A CMP tool of the prior art, shown in simplified form in FIG. 1, illustrates a substrate 78 held by a tooling head 66 which rotates about the central axis of the substrate. A circular polishing pad 60 is rotated while in contact with the bottom surface of the rotating substrate. The rotating substrate contacts the larger rotating polishing pad 60 in an area away from the center of the polishing pad. A slurry dispense nozzle 61 positioned above the surface of the polishing pad dispenses a slurry 63, containing an abrasive and at least one chemically-reactive agent, on the polishing pad 60 by way of a supply circuit, (not shown) and carried to the interface between the polishing pad 60 and substrate. A polishing pad dressing head 67 is pressed downward 69 and oscillates against the top surface of the polishing pad 60 to restore the texture to the polishing pad, thereby, preventing a glaze-like build up of slurry during and after polishing.
The problem with this method of polishing is that many of the oxides deposited on the wafer, by plasma enhanced methods, are thicker at the wafer edge. The thickness variance could measure upwards to 1000 angstroms. This is a continuing process control problem that needs a method of polishing that would quicken the polishing rate at the thicker edge and at the same time slowing the polishing rate towards the center of the wafer.
In view of the above problem, there is a need to improve the method of planarizing when using the CMP process. It is therefore an object of the present invention to provide a slurry dispensing apparatus for a chemical mechanical polishing machine that does not have the drawbacks or shortcomings of the conventional slurry dispensing methods.
It is another object of the present invention to provide a slurry dispensing apparatus for a chemical mechanical polishing machine that is provided with a slurry manifold having a plurality of nozzles each of which would radially distribute different solids to liquid concentrations.
It is yet another object of the present invention to allow a tailoring of the oxide polishing rate across the wafer. Unlike the conventional diaphragm type polishing heads, where zone polishing is not offered due to its fixed physical characteristics.
It is still another object of the present invention to allow the user to have unlimited control of the polishing rate on the wafer from its center to its peripheral edge, therefore, providing better polishing uniformity to the varying topography of the wafer.